The present invention relates to a rechargeable phase change material unit and a food heating device employing such a unit. Once charged with heat energy, a phase change material in the phase change material unit radiates heat energy to keep objects and foods warm.
For food delivery, insulated boxes and thermal bags are currently used to keep food warm while it is transported from one place to another. Typically, these bags and boxes also include a heat reflective material on their inner surfaces. Even though the food is stored in an insulated box or bag, the temperature of the food begins declining once the food is removed from the cooking device. Thermal bags and boxes only serve to reduce the rate at which the food loses heat and do not maintain the temperature of the food.
Phase change materials have been found to be particularly useful for food warming, especially during transport, because they utilize their latent heats of fusion for thermal storage. The latent heat of fusion of a phase change material is substantially greater than the sensible heat capacity of the material. Upon melting and freezing, per weight, a phase change material absorbs and releases substantially more energy than a sensible heat storage material which is heated and cooled in the same temperature range. In contrast to a sensible heat storage material which absorbs and releases heat energy essentially uniformly over a broad range of temperature, a phase change material absorbs and releases a large quantity of energy in the vicinity of its melting/freezing point in a controlled manner. In addition to its latent heat storage capacity, a phase change material also stores and releases sensible heat.
Various food warming devices containing phase change materials have been proposed. See, for example, the following U.S. Pat. Nos.: 4,246,884; 4,982,722; 5,254,380; and 5,520,103. However, most of these devices are designed so that the phase change material is charged by heating the device in a convection oven. When heated in this manner, a lengthy period of time is required to charge the phase change material. Further, once these devices have been heated, they continue to lose heat as the phase change material discharges its stored heat energy.
A device having an electric heating element sandwiched between two cross-linked high density polyethylene (HDPE) sheets is currently available from Vesture Corporation (the Vesture Heat Storage Board). In some instances, a plastic, such as HDPE, can be considered to be a phase change material. However, the maximum latent heat of such a product occurs at about 132.degree. C., a temperature that is too high for hot food serving since it might dry or char the food and could burn the people handling the food. At the lower temperature of about 60.degree. C. desired for hot food serving, the Vesture Heat Storage Board will only store and release the sensible heat of the cross-linked HDPE which produces only about 6 calories over a temperature range of 10.degree. C. This value is very low and is, even then, available for only a short period of time (determined by the insulation) because cross-linked HDPE has little capacity for thermal energy storage. For these reasons, it appears that the cross-linked HDPE plates or sheets are simply being used in the Vesture Heat Storage Board has a cover to protect an electric heating element which is the source of the energy for food warming purposes.
A need remains in the art for a food warming device which employs a phase change material, which emits heat for longer periods of time than currently used devices, and which can be easily recharged while simultaneously being used to warm a food.